Multilayer composite pressure vessel and method for making the same

ABSTRACT

A method for manufacturing a pressure vessel includes the steps of obtaining a metal liner and applying a first layer of adhesive to an outer surface of the metal liner. The metal liner is then placed into a vacuum bag and a vacuum is applied, thus securing the adhesive to the outer surface of the metal liner. After curing of the adhesive, a filamentary material is wound around the liner, where the filamentary material adheres to the first layer of adhesive forming an overwrap layer on the outer surface of the metal liner, forming the pressure vessel.

FIELD OF THE INVENTION

The present invention relates to containers in general. Moreparticularly, the present invention relates to light weight vesselscapable of accommodating various media at relatively high pressures.

BACKGROUND OF THE INVENTION

In many technical fields, a need exists for storing various liquid orgaseous media, such as compressed or liquefied gases, for extendedperiods of time and frequently at very high pressures. Many attemptshave already been made in the past to satisfy this need by developinglightweight pressurized medium containers or pressure vessels that wouldaccommodate the pressurized medium without suffering leakage losses orstructural damage.

For a variety of reasons, not the least important of which is therelatively high ratio of pressure that the vessel walls are able towithstand to the weight of a vessel of a given capacity, it has beenfound advantageous to give such walls a multilayer or compositestructure, including an inner liner and an outer shell surrounding theliner and in intimate contact therewith. The liner is formed of amaterial usually a metallic material that is compatible with (i.e. inertwith respect to) and also completely or at least highly impermeable tothe medium being stored.

All-metallic pressure vessels have been disclosed, for example, in U.S.Pat. Nos. 2,127,712; 2,661,113; 3,140,006; and 4,964,524, of which allbut the second one are directed to vessels of multilayer construction.In this instance, one of the purposes of the liner is to form an inertprotective barrier preventing the medium from reaching through grossleakage or permeation through the liner to the outer shell and possiblydamaging the shell. However, due to their considerable thickness andintimate contact or engagement with the shell, the liners ofall-metallic pressure vessels generally contribute significantly to theload bearing capacity of the vessel. In classical state of the artvessel fabrication, the liner represents a significant fraction of thetotal weight. Experience with such and similar all-metallic pressurevessel constructions has shown, on the other hand, that they are limitedin applicability because they are either too heavy (a criterion that isof paramount importance for applications where weight is at a premium,such as in outer space applications), or expensive to manufacture, orprone to failure, especially due to metallic material fatigue atweakened or stress concentration regions after having been subjected toa number of pressurization and depressurization cycles.

With the advent and development of high strength filaments such asglass, graphite, and synthetic plastic material fibers, and ofmaterials, such as epoxy resins, capable of forming a matrix embeddingsuch filaments and bonding them together into a composite structure,attempts have been made, some more successful than others, to use suchcomposite materials for the outer shell of the pressure vessel. Ofcourse, due to the high strength-to-weight ratio of such materials, theoverall weight of the resulting vessel is significantly reduced relativeto that of a comparable all-metallic vessel of the same capacity andpressure rating. Examples of vessels of this kind are disclosed, forexample, in U.S. Pat. Nos. 2,744,043; 2,827,195; 3,943,010; 3,969,812,4,040,163; and 5,653,358.

For example, U.S. Pat. No. 5,653,358 among other elements, describes atank of composite structure. A vessel is comprised principally of aninner liner (such as a metal liner) coated with a primer and an overwrapor jacket. To that end, the outer jacket is constructed, in a knownmanner, by superimposed and overlapping layers of impregnatedfilamentary material that contains glass, graphite or Kevlar™ fiberswrapped in different directions around the liner, with the intersticesbetween the fibers or filaments being filled by impregnating materialsuch as hardenable epoxy resin that, upon setting or hardening, forms amatrix that firmly embeds such fibers or filamentary material.

Thus, after hardening, the filamentary and impregnating materialtogether form a composite, fiber reinforced solid body that is capableof withstanding most if not all of the forces applied to the vesselduring its lifetime.

However, the prior art methods of applying the wrapped jacket to themetal liner of the vessel suffers from many drawbacks. In the prior art,a metal liner is first coated with a primer and then the adhesive isused to structurally couple the liner to the overlying filament woundcomposite. Most commonly a reticulating film adhesive is used. In suchan application, pre-cut pattern shapes or gore panels are applied bygloved hands prior to the commencement of composite lay-up. In thisapplication, fibers are impregnated with a wet winding resin beforeapplication. The bearing pressure for bondline curing, required toensure a good bond between the adhesive and the metal liner, isdeveloped as a by-product of the tension in the fiber tows (jacketmaterial) that are filament wound in a pre-programmed repeating closurepattern over the liner and adhesive.

Upon completion of the winding, which consists of multiple layers of thereinforcing fibers and impregnating resin, the composite structure iscured. The adhesive and wet winding resins are compatible andco-curable.

This wet winding process by its nature develops variable bearingpressures on the liner as a result of fiber buildup near the polarregions (ie. boss and/or exit of the vessel) of the wind. Thisfrequently results in roping/bridging of the fiber with a resulting lossof bearing pressure. The bearing pressure also varies due to resinrheology (time and temperature dependent viscosity response.) Resintrough behavior is recognized as resin bleedout during the early stagesof wet winding cure (first stage also known as gelation). Thevariability in bearing pressure is a limiting factor in developingoptimal adhesion of the overwrap layer to the metal liner. The criticalparameter is adhesion to the metal liner, the adhesion to the co-curedoverlying composite structure is readily achieved through selection ofcompatible and co-curable adhesives and wet winding epoxies.

It is understood, that structural coupling between the inner liner layerand the outer jacket layer is critical, particularly when the pressurevessels are filled with loads under high pressure. Poor adhesion betweenthe overwrap layer and the metal liner caused by irregular bearingpressure during the overwrap application can result in liner elasticstability/buckling failure. The thin metal liner, as a standalonestructural entity, is incapable of supporting the high bearing pressureimposed at zero or low pressure by the overlying composite after vesselautofrettage. The elastic stability or buckling failure of the linerresults in dramatic reductions in fatigue life, resulting through cracksin the metal liner and leakage of contents within a small number ofcycles.

OBJECTS AND SUMMARY OF THE INVENTION:

As such it is the object of this invention to provide a high pressurelight-weight pressure vessel that overcomes the drawbacks associatedwith the prior art and is constructed having improved adhesion betweenthe inner liner and outer jacket layers. Additionally, it is an objectof the present invention to provide a method of consistentlymanufacturing such pressure vessels with improved adhesion between theinner liner and outer jacket layers.

To this end, the present invention provides a method for manufacturing ahigh pressure light-weight pressure vessel including the steps ofobtaining a metal liner, applying a first layer of adhesive to an outersurface of the metal liner, placing the metal liner into a vacuum bagand applying a vacuum to the vacuum bag, thus securing the adhesive tothe outer surface of the metal liner. After curing of the adhesive, afilamentary material is wound around the liner, where the filamentarymaterial adheres to the first layer of adhesive forming an overwraplayer on the outer surface of the metal liner, forming the pressurevessel.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with features, objects, and advantages thereof may best beunderstood by reference to the following detailed description when readwith the accompanying drawings:

FIG. 1 is a cross section of a pressure vessel, according to oneembodiment of the present invention;

FIG. 2 is a close up the filament of the overwrap layer, in accordancewith one embodiment of the present invention;

FIG. 3 is a flow chart depicting the process for forming the pressurevessel from FIG. 1, in accordance with one embodiment of the presentinvention;

FIG. 4 is a close up of the adhesive layer being applied to the liner,in accordance with one embodiment of the present invention; and

FIG. 5 is a view of the adhesive layer being cured to the liner of thepressure vessel by a vacuum bag, in accordance with one embodiment ofthe present invention.

DETAIL DESCRIPTION

Referring now to the drawings, and initially to FIG. 1, it may be seenthat the general reference numeral 10 identifies a pressure vesselembodying the present invention. Pressure vessel 10 (also referred to asvessel 10) has a main portion 11 that bounds an internal chamber orinterior 12 of vessel 10, and a stem or neck portion 13 that projectsout of the main portion 11 along an axis and is hollow to define apassage 14 for establishing communication between the interior 12 of thevessel 10 and its exterior.

As illustrated in FIG. 1, the vessel 10 is of a multilayer or compositestructure in that it includes an inner liner 15 and an outer jacket oroverwrap 16 that surrounds the liner 15 and, more particularly, at leasta main portion 11 of the liner that bounds interior 12. Liner 15 used inthe vessel 10 of the present invention is preferably very thin, suchthat its thickness is chosen to be just above the minimum needed toprevent permeation of the medium contained in interior 12 through liner15 at the highest pressure differential expected to be encounteredbetween interior 12 of vessel 10 and its exterior during the lifetime ofvessel 10, and at a level needed to prevent tearing of or other physicaldamage to liner 15 when exposed to the highest anticipated or intendedinternal pressure.

Liner 15 is a thin metal element formed via seamless spinning, weldingof a formed and machined part, electrodeposition or other suchtechniques. The diameter to thickness ratio of the liner may be 500/1and above. The thickness of liner 15 generally is insufficient to enableliner 15 to withstand the expected internal pressures on its own or evento make more than a rather insignificant (less than 5%) contribution tothe overall strength of vessel 10. More importantly, without a highintegrity coupling to a overlying composite 16 described below, thecompressive stresses in the metal liner 15, developed in reaction to theimposed bearing from the pre-stressed overlying composite at the zeropressure condition typically causes and elastic stability or bucklingfailure. At operating pressure the overwhelming majority of the load isborne by the outer jacket 16. Both liner 15 and composite 16 are in astate of tension.

As illustrated in FIG. 2, a close up of a single fiber or filamentarymaterial 50 from overwrap layer 16 is shown. Overlay layer 16 istypically made from superimposed and overlapping layers of thisimpregnated filamentary material 50 that contains glass, graphite orKevlar™ fibers 52 wrapped in different directions around liner 15, withthe interstices 54 between the fibers or filaments being filled byimpregnating material such as hardenable epoxy resin that, upon settingor hardening, forms a matrix that firmly embeds such fibers orfilamentary material.

Turning now to the construction process of vessel 10, a flow chart isshown in FIG. 3, illustrating a typical step by step process forapplying overwrap 16 to liner 15. It is understood that the followingprocess is exemplary of the salient features of the process, but thatcertain steps may either be added, eliminated moved or otherwisealtered, provided that the essential steps are all included.

At a first step 100, the metal liner 15 is subjected to a media blastingoperation for surface roughening and/or mechanical surface activation.This step is to ensure that the adhesive discussed in detail below hasimproved surface area for maximum binding with the outer surface ofliner 15. Media blasting may take the form of glass bead, Al (Aluminum)Oxide or other conventional media as used to impart random roughening toa surface.

Next, at step 102, the media blasted liner 15 is treated with an acidwash for cleaning away all molecular/organic contaminants and surfaceresident blasting particles and to provide chemical activation of thesurface. This is accomplished by different acid mixtures depending onliner material. Nitric acid is a common constitute and is widely usedfor aluminum liners. Fluoric or oxalic acid additions are made fornickel or titanium based alloys. The intent of media blasting, followedby acid washing is to provide a roughened surface free of detrimentaloxide films and any molecular contamination that could ultimatelycontribute to reduced bond strength. The liner is then water rinsed toremove all acid residue.

After cleaning, at step 104, a water break test is performed to thedried liner. Distilled water is sprayed against liner 15. If it isclean, the water should sheet off of the part. If not clean, the watertends to bead. If liner 15 does not pass the water test it may bereturned to step 102 and washed again until clean.

Assuming the part is clean, at step 106, a film adhesive layer 20 isapplied to liner 15, as shown in close up illustration FIG. 4. Thus,rather than using a typical primer as done in the prior art, whichprovides inferior structural response properties to the adhesive, thepresent invention, binds the adhesive directly to the metallic outsideof liner 15. This adhesive layer 20 is applied to the outside of theliner 15 by the conventional method using pre-cut pattern shapes or gorepanels, by gloved hands.

At step 108, peel ply cloth 21, is applied to the uncured adhesive 20.Removal of peel ply 21, post cure, provides optimal surface roughness. Asecond purpose of peel ply 21 is to provide a direct contact breatherelement, allowing volatiles from the film adhesive 20 to de-gas duringconsolidation and cure without leaving bubble artifacts post cure as areoften seen in vacuum bagged cured film adhesive.

Next, at step 110, a release film 22 is applied to the peel ply 21, aswell as a breaker cloth 24, both shown in FIG. 4. The purpose of releasefilm 22 is to allow parting or disassembly upon completion of cure.Breather cloth 24, typically constructed of coarse weave fiberglassserves to provide a bearing distribution path from vacuum bag 30,discussed below, onto release film 22, further transmitting onto peelply 21, film adhesive 20, and ultimately metal liner 15, thus providinguniform bearing pressure during cure.

After adhesive layer 20, peel ply 21, release film 22 and breaker cloths24 are applied, at step 112, a vacuum bag 30 is placed over the entirecoated liner 15. As with typical vacuum bag processes liner 15 is placedin vacuum bag 30, which is then evacuated, thus by bag pressure,providing a pressurized cure environment for adhesive 20. Vacuum bag 30is evacuated through a fitting in the bag via a vacuum pump 32. Thisvacuum bag process is most typically used to de-gas/consolidate, andsubsequently cure hand layup pre-impregnated fibrous compositesstructures. De-gassing can also be referred to as de-bulking as it worksto reduce the void fraction in the layed-up composite structure.

The vacuum process may act as both a de-bulking process and a cureprocess. Typically, however heat is applied to cure the adhesive 20 toperform the curing. operation. Positive pressure cure ovens(autoclaves), which will operate at 25 to 75 psid are commonly used.This allows release of the internal vacuum from bag 30 (venting theinside of the bag to atmospheric pressure), while maintaining a pressuredifferential across bag 30 by virtue of pressurization of theoven(autoclave). The venting of the inside of bag 30 is most often doneto prevent rapid volatilization of impregnating resin or adhesiveconstituents, which will result in reduced composite properties due tohigh void fraction (porosity.)

In the present invention, a typical heat the vacuum process may employ aramping up and down of the curing temperature. On such example, wouldfollow the following steps: 1) vacuum to 25 inches of Mercury or better;2) hold for 10 minutes; 3) ramp up temperature to 250° F. atapproximately 2 to 5° F./minute; 4) hold at 250° F. +/−10° F. for 90+/−15 minutes; ramp down temperature at approximately 2 to 5° F./minute.It is understood that this is just one example of temperatures used forcuring, however other temperatures may be used as necessary fordifferent adhesives 20.

In the present invention, the need to vent bag 30 for curing operationsis eliminated by two factors. The first factor being that peel ply 21placed in intimate contact with the thin layer of adhesive 20 serves thefunction of a secondary breather element. There is 100% surface contactto the thin underlying adhesive 20 to peel ply 21 giving adhesivevolatiles a low resistance path for evacuation. Thus, large detrimentalvoids are not created. Secondly, the small voids in the thin curedadhesive film 20 will be open to the surface upon removal of peel ply21. The second layer of film adhesive included in impregnatedfilamentary material 50 upon application of the winding 16 flows intothe open surface voids providing beneficial mechanical interlockingbetween adhesive layer 20 and filamentary material 50 of overwrap layer16.

Once the vacuum step is complete, at step 114, the cured adhesive layer20 is, if required, treated by sanding or scuffing to remove anywrinkles, leaving the textured peel ply surface in the adhesive 20 as isachieved through the stripping (removal of the peel ply 21.

Next, at step 116, once adhesive layer 20 is prepared and cured, overlaplayer 16 is then applied to the coated liner 15. As discussed above,overlapping layers of impregnated filamentary material 50 that containsglass, graphite or Kevlar™ fibers are wrapped in different directionsaround liner 15, with the interstices 52 between the fibers or filamentsbeing filled by impregnating material 54 such as hardenable epoxy resin(wet winding resin) that, upon setting or hardening, forms a matrix thatfirmly embeds such fibers or filamentary material 50. The filament 50 iswound in a pre-programmed repeating closure pattern over the liner andadhesive layer 20 of liner 15. This results in a thorough and completebinding between liner 15 and overwrap layer 16.

The above described process provides a distinct advantage over the priorart. First, consistent high strength bonds are readily achieved on epoxycompatible epoxy substrates than metal substrates. Roughened epoxysubstrates, such as a peel ply surface, represent the ideal substrate.

Thus, by using the vacuum process outlined above for adhesive layer 20,a solid and complete bond between the metal liner 15 and adhesive 20 isformed. The pressure of the vacuum ensures this process significantlybetter than in prior art systems where the pressure to cause adhesionbetween the adhesive 20 and metal liner 15 was only a by-product resultof the uneven and unpredictable binding pressure of the overwrap layer16. Bonding to the metal surface of liner 15 (substrate) is done atknown, consistent and verifiable bearing pressure by the above vacuumprocess.

Thereafter, the present invention allows for overwrap layer 16, with itsimpregnated wet winding resin, to be applied to a cured adhesive layer20, having an excellent roughened binding surface as well as a fullyconnected and cured adhesion to the metal surface of liner 15.

Splitting the operation into two steps, vacuum and cure of adhesivelater 20 and winding of overwrap layer 16, dramatically enhances theadhesion of overwrap layer 16 to the metal substrate (liner 15). Thevacuum bag de-bulking insures removal of entrapped air between metalliner 15 and adhesive 20. A consistent bearing pressure as imposed bythe vacuum bag 30 maximizes flow of adhesive 20 into the micromechanicalvalleys of the random roughness prepared metal substrate (liner 15). Theconsistent bearing pressure cannot be achieved in traditionalprocessing. After cure the integrity of the adhesive to liner binding(cohesion) can be verified through both the mechanical stripping of thepeel ply 21, which provides an in-process peel test, to screensubstandard bonding and through visual inspection upon removal of peelply 21.

The overall adhesion of overwrap layer 16 to metal liner 15 is thefabrication critical element for vessel 10. Far less bearing pressure isrequired to develop a high integrity bond to a compatible epoxysubstrate than to a primed metal liner of the prior art. The direct andcontinuous processing through surface isolation of metal liner 15 byadhesive layer 20 is accomplished through the above process. The curedadhesive layer 20 serves as a protective primer, with superiorperformance to conventional brush on or spray on primers.

The epoxy substrate (adhesive layer 20) from the above outlined vacuumprocess is rougher. At a micromechanical level there is open surfaceporosity, which allows a second layer of adhesive (epoxy impregnatedinto the filamentary material 50) to flow into and cure within themicrovoids during step 116 when overwrap layer 16 is applied. Thisprovides a mechanical interlocking to the substrate (adhesive layer 15)resulting in a high fidelity bond.

Further steps may be taken to improve the overall quality of vessel 10.For example, witness coupons for the critical bonding process can beprepared and tested and accurately reflect the peel performance to metalliner 15. Post operation visual inspection is readily performed as wellas removal and rework of substandard cure, readily achieved by nitricacid digestion.

While only certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes orequivalents will now occur to those skilled in the art. It is therefore,to be understood that this application is intended to cover all suchmodifications and changes that fall within the true spirit of theinvention.

1. A method for manufacturing a pressure vessel, said method comprisingthe steps of: obtaining a metal liner; applying a first layer ofadhesive to an outer surface of said metal liner; placing said metalliner into a vacuum bag; applying a vacuum to said vacuum bag, thussecuring said adhesive to the outer surface of said metal liner; aftercuring of said adhesive, winding a filamentary material around saidliner, wherein said filamentary material adheres to said first layer ofadhesive forming an overwrap layer on the outer surface of said metalliner, forming said pressure vessel.
 2. The method as claimed in claim1, further comprising the step of performing an abrasion to said metalliner, prior to applying said first layer of adhesive.
 3. The method asclaimed in claim 1, further comprising the step of applying a peel plyover said first layer of adhesive.
 4. The method as claimed in claim 3,further comprising the step of applying a release film on top of saidpeel ply.
 5. The method as claimed in claim 4, further comprising thestep of applying a breather cloth on top of said release film.
 6. Themethod as claimed in claim 1, where said step of applying a vacuumfurther comprises the step of heating said contents within said vacuumbag.
 7. The method as claimed in claim 6, wherein said step of heatingsaid contents within said vacuum bag is performed by graduallyincreasing the temperature to substantially 250° F. at a rate ofsubstantially 7° F. per minute.
 8. The method as claimed in claim 5,wherein after removing said vacuum bag, before applying said winding,said release film, breather cloth and peel ply are removed, said removalof said peel ply causing a textured surface to appear on said firstadhesive layer.
 9. The method as claimed in claim 1, wherein afterremoving said vacuum bag, before applying said winding said firstadhesive layer is cleaned.
 10. The method as claimed in claim 1, whereinsaid winding formed from said filamentary material is impregnated withresin that binds with said first adhesive layer to form said pressurevessel.
 11. A pressure vessel for containing materials under elevatedpressures, said pressure vessel comprising: a metal liner; an adhesivelayer, applied to the outer surface of said metal liner, wherein saidadhesive layer is treated with a vacuum bag in order to secure saidadhesive to the outer surface of said liner; and an overwrap layerapplied on top of said adhesive on the outer surface of said metalliner, wherein said overwrap layer is formed by winding a filamentarymaterial around said liner, such that the filamentary material adheresto said adhesive forming an overwrap layer on the outer surface of saidmetal liner, forming said pressure vessel.
 12. The system as claimed inclaim 11, wherein said metal liner is treated with abrasion prior toapplying said first layer of adhesive.
 13. The system as claimed inclaim 11, further comprising a peel ply applied over said first layer ofadhesive.
 14. The system as claimed in claim 13, further comprising arelease film applied on top of said peel ply.
 15. The system as claimedin claim 14, further comprising a breather cloth applied on top of saidrelease film.
 16. The system as claimed in claim 15, wherein said peelply causes a textured surface to appear on said first adhesive layer.17. The system as claimed in claim 11, wherein said winding formed fromsaid filamentary material is impregnated with resin that binds with saidfirst adhesive layer to form said pressure vessel.